linux/fs/fs_context.c

/* Provide a way to create a superblock configuration context within the kernel
 * that allows a superblock to be set up prior to mounting.
 *
 * Copyright (C) 2017 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
#include <linux/fs_context.h>
#include <linux/fs.h>
#include <linux/mount.h>
#include <linux/nsproxy.h>
#include <linux/slab.h>
#include <linux/magic.h>
#include <linux/security.h>
#include <linux/mnt_namespace.h>
#include <linux/pid_namespace.h>
#include <linux/user_namespace.h>
#include <net/net_namespace.h>
#include "mount.h"
#include "internal.h"

struct legacy_fs_context {
	char			*legacy_data;	/* Data page for legacy filesystems */
	size_t			data_size;
};

static int legacy_init_fs_context(struct fs_context *fc);

/**
 * alloc_fs_context - Create a filesystem context.
 * @fs_type: The filesystem type.
 * @reference: The dentry from which this one derives (or NULL)
 * @sb_flags: Filesystem/superblock flags (SB_*)
 * @sb_flags_mask: Applicable members of @sb_flags
 * @purpose: The purpose that this configuration shall be used for.
 *
 * Open a filesystem and create a mount context.  The mount context is
 * initialised with the supplied flags and, if a submount/automount from
 * another superblock (referred to by @reference) is supplied, may have
 * parameters such as namespaces copied across from that superblock.
 */
static struct fs_context *alloc_fs_context(struct file_system_type *fs_type,
				      struct dentry *reference,
				      unsigned int sb_flags,
				      unsigned int sb_flags_mask,
				      enum fs_context_purpose purpose)
{
	int (*init_fs_context)(struct fs_context *);
	struct fs_context *fc;
	int ret = -ENOMEM;

	fc = kzalloc(sizeof(struct fs_context), GFP_KERNEL);
	if (!fc)
		return ERR_PTR(-ENOMEM);

	fc->purpose	= purpose;
	fc->sb_flags	= sb_flags;
	fc->sb_flags_mask = sb_flags_mask;
	fc->fs_type	= get_filesystem(fs_type);
	fc->cred	= get_current_cred();
	fc->net_ns	= get_net(current->nsproxy->net_ns);

	switch (purpose) {
	case FS_CONTEXT_FOR_MOUNT:
		fc->user_ns = get_user_ns(fc->cred->user_ns);
		break;
	case FS_CONTEXT_FOR_SUBMOUNT:
		fc->user_ns = get_user_ns(reference->d_sb->s_user_ns);
		break;
	case FS_CONTEXT_FOR_RECONFIGURE:
		/* We don't pin any namespaces as the superblock's
		 * subscriptions cannot be changed at this point.
		 */
		atomic_inc(&reference->d_sb->s_active);
		fc->root = dget(reference);
		break;
	}

	/* TODO: Make all filesystems support this unconditionally */
	init_fs_context = fc->fs_type->init_fs_context;
	if (!init_fs_context)
		init_fs_context = legacy_init_fs_context;

	ret = init_fs_context(fc);
	if (ret < 0)
		goto err_fc;
	fc->need_free = true;
	return fc;

err_fc:
	put_fs_context(fc);
	return ERR_PTR(ret);
}

struct fs_context *fs_context_for_mount(struct file_system_type *fs_type,
					unsigned int sb_flags)
{
	return alloc_fs_context(fs_type, NULL, sb_flags, 0,
					FS_CONTEXT_FOR_MOUNT);
}
EXPORT_SYMBOL(fs_context_for_mount);

struct fs_context *fs_context_for_reconfigure(struct dentry *dentry,
					unsigned int sb_flags,
					unsigned int sb_flags_mask)
{
	return alloc_fs_context(dentry->d_sb->s_type, dentry, sb_flags,
				sb_flags_mask, FS_CONTEXT_FOR_RECONFIGURE);
}
EXPORT_SYMBOL(fs_context_for_reconfigure);

struct fs_context *fs_context_for_submount(struct file_system_type *type,
					   struct dentry *reference)
{
	return alloc_fs_context(type, reference, 0, 0, FS_CONTEXT_FOR_SUBMOUNT);
}
EXPORT_SYMBOL(fs_context_for_submount);

void fc_drop_locked(struct fs_context *fc)
{
	struct super_block *sb = fc->root->d_sb;
	dput(fc->root);
	fc->root = NULL;
	deactivate_locked_super(sb);
}

static void legacy_fs_context_free(struct fs_context *fc);

/**
 * put_fs_context - Dispose of a superblock configuration context.
 * @fc: The context to dispose of.
 */
void put_fs_context(struct fs_context *fc)
{
	struct super_block *sb;

	if (fc->root) {
		sb = fc->root->d_sb;
		dput(fc->root);
		fc->root = NULL;
		deactivate_super(sb);
	}

	if (fc->need_free && fc->ops && fc->ops->free)
		fc->ops->free(fc);

	security_free_mnt_opts(&fc->security);
	put_net(fc->net_ns);
	put_user_ns(fc->user_ns);
	put_cred(fc->cred);
	kfree(fc->subtype);
	put_filesystem(fc->fs_type);
	kfree(fc->source);
	kfree(fc);
}
EXPORT_SYMBOL(put_fs_context);

/*
 * Free the config for a filesystem that doesn't support fs_context.
 */
static void legacy_fs_context_free(struct fs_context *fc)
{
	kfree(fc->fs_private);
}

/*
 * Add monolithic mount data.
 */
static int legacy_parse_monolithic(struct fs_context *fc, void *data)
{
	struct legacy_fs_context *ctx = fc->fs_private;
	ctx->legacy_data = data;
	if (!ctx->legacy_data)
		return 0;
	if (fc->fs_type->fs_flags & FS_BINARY_MOUNTDATA)
		return 0;
	return security_sb_eat_lsm_opts(ctx->legacy_data, &fc->security);
}

/*
 * Get a mountable root with the legacy mount command.
 */
static int legacy_get_tree(struct fs_context *fc)
{
	struct legacy_fs_context *ctx = fc->fs_private;
	struct super_block *sb;
	struct dentry *root;

	root = fc->fs_type->mount(fc->fs_type, fc->sb_flags,
				      fc->source, ctx->legacy_data);
	if (IS_ERR(root))
		return PTR_ERR(root);

	sb = root->d_sb;
	BUG_ON(!sb);

	fc->root = root;
	return 0;
}

/*
 * Handle remount.
 */
static int legacy_reconfigure(struct fs_context *fc)
{
	struct legacy_fs_context *ctx = fc->fs_private;
	struct super_block *sb = fc->root->d_sb;

	if (!sb->s_op->remount_fs)
		return 0;

	return sb->s_op->remount_fs(sb, &fc->sb_flags,
				    ctx ? ctx->legacy_data : NULL);
}

const struct fs_context_operations legacy_fs_context_ops = {
	.free			= legacy_fs_context_free,
	.parse_monolithic	= legacy_parse_monolithic,
	.get_tree		= legacy_get_tree,
	.reconfigure		= legacy_reconfigure,
};

/*
 * Initialise a legacy context for a filesystem that doesn't support
 * fs_context.
 */
static int legacy_init_fs_context(struct fs_context *fc)
{
	fc->fs_private = kzalloc(sizeof(struct legacy_fs_context), GFP_KERNEL);
	if (!fc->fs_private)
		return -ENOMEM;
	fc->ops = &legacy_fs_context_ops;
	return 0;
}

int parse_monolithic_mount_data(struct fs_context *fc, void *data)
{
	int (*monolithic_mount_data)(struct fs_context *, void *);
	monolithic_mount_data = fc->ops->parse_monolithic;
	return monolithic_mount_data(fc, data);
}